Materials change when I export a fbx file to Unity
This is normal, occurs because each software has a different material system. 

When you export to fbx it converts the information to a "universal" compatibility format, so some types of standard information are preserved, like diffuse color, vertex paint, UV Maps, but not Blender specific settings like multi layered textures, alpha, blending mode, etc. . 

Unity has a very restrictive material system that makes hard to adapt complex graphic ports. In some instances you may have even to write shaders in c# code to get good results, so for porting to Unity keep your materials with as simple setup as possible.